This application claims the priority of German Patent Document 100 08 299.8, filed Feb. 23, 2000, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a method for operating a three-phase machine in a vehicle.
In the case of a concentric drive train generator, an electric machine is installed between the internal combustion engine and the gearbox which can both start the internal combustion engine and ensure the power supply of the vehicle.
However, for a given installation space and given battery power the output torque of the electric machine is frequently insufficient at zero speed to start large-volume spark-ignition engines and, specifically, diesel engines at low temperatures.
Various methods are known for solving this problem. As an example, it has been proposed in the case of an inertia start for a flywheel mass accelerated by the electric machine to be suddenly coupled to the internal combustion engine in order to start the latter moving. This necessitates a clutch with an actuator and controller, which require additional installation space. In the case of a system without a clutch, with large engines there is also a need for conventional starters with drive circuits, which are required only for the case of cold starting.
Two fundamentally opposing requirements are to be fulfilled in the case of electric machines which simultaneously form the starter and the generator for a motor vehicle. Firstly, it is necessary for starting to apply a particularly high breakaway torque which, depending on the displacement and/or number of cylinders and type of the internal combustion engine, for example spark-ignition engine or diesel engine, and depending on the temperatures, can greatly vary. There is also a need, moreover, to apply torque reserves in order to accelerate the internal combustion engine to starting speed.
Secondly, after the internal combustion engine has been started, the electric machine, designed as a starter/generator, is to operate as a generator in order to feed the motor vehicle network. In this case, there is a need for a power output which is as constant as possible over the wide speed range prescribed by the internal combustion engine, usually being 600 to 6000 RPM, in conjunction with an efficiency which is as high as possible.
These two requirements are difficult to satisfy economically with a standard drive, for example comprising a three-phase polyphase machine and a voltage-injecting pulse-controlled inverter in a three-phase bridge connection. If such an electric combination machine is designed for torque, the stator coils contain a relatively large number of turns. The power output of such a machine is, however, already limited at medium speed. Because of the many windings, field weakening operation begins relatively early, and so there is not sufficient control margin available for the pulse-controlled inverter.
If, by contrast, the machine is designed for generator power, the number of turns is correspondingly lower. However, this allows a corresponding rise in the phase currents required to generate the starting torque. A particular problem resides therefore in the overall size of the pulse-controlled inverter, which must permanently exert control over the currents. Furthermore, the battery is subjected to higher loading since high currents are required during starting. In addition, there is a problem with generating sufficient electric power during idling, that is, of displacing the so-called start-up speed or initiation speed of the machine as far as possible into the range of low speeds. This results in a limitation of the power output at higher speeds. Although a corresponding power excess occurs in theory, it cannot be used, because of the fact that the vehicle network voltage is clamped at a fixed value, for example 14 V.
Another method consists in optimizing the operating modes of the electric machine to the effect that at low speeds below the idling speed of the internal combustion engine an operating mode which is optimal for outputting torque is selected, for example a star connection, while for generator operation of the electric machine in the remaining speed band an operating mode is selected which is optimized for efficiency, for example a delta connection. In this case, it is imperative for a traction drive that the operating mode is switched over without a torque jump in simultaneous conjunction with uninterrupted output of torque. Such a method has been proposed in DE 197 33 208 C1 for a generator.
The object of the invention is to specify a method for operating a three-phase machine which permits simplified switchover of operating mode.
In accordance with the invention, switching over from one method of connection to the other is performed at an instant at which the three-phase machine changes from the state of picking up torque to outputting torque, or from the state of outputting torque to picking up torque.
The advantage is to be seen in that changes in the torque/current characteristic have no effect during switchover, since at this instant the torque is equal to zero. In addition, electromechanical switching elements in the system, which are provided to guard against short circuits, are not subjected to loading by high currents during switchover, and can therefore be of simpler and less expensive design.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.